Symbiotic characteristics of Rhizobium meliloti in west asian soils

Abstract

Experiments conducted with 247 isolates of Rhizobium meliloti from soil samples collected from diverse arable areas in west Asia (Cyprus. Jordan. Lebanon, Syria and Turkey) were assessed to define their symbiotic characteristics and degree of host specificity on six species of annual Medicago (M. rigidula (L.) All., M. nocana (Boiss), M. rotata (Boiss), M. orbicularis (L.) Bart. and Australian cullivars of M. truncatula Gaertn. cv. Jemalong and M. polymorpha L. cv. Circle Valley). Host species responses were evaluated based on nodule appearance and plant biomass. The test soils contained populations of R. meliloti with differing ability to nodulate these specific legumes and fix N2. The isolates were classified into four categories: highly effective, partially effective, ineffective and non-nodulating. The number of R. meliloti in the soil was < 7.2 × 105 g1−. Soils from Syria. Jordan and Turkey contained higher proportions of ineffective rhizobia than those from Cyprus and Lebanon. Despite the widespread presence of ineffective rhizobia. highly-effective strains were identified from all soils and all countries for each of the six species of Medicago. The proportion of highly-effective rhizohia nodulating all hosts was < 10% of the total rhizobial population in the samples from each country. A significantly higher proportion of partially-effective rhizohia was present in most samples for each host.